HIGH-LATITUDE, SUMMERTIME NOX ACTIVATION AND SEASONAL OZONE DECLINE IN THE LOWER STRATOSPHERE - MODEL-CALCULATIONS BASED ON OBSERVATIONS BYHALOE ON UARS

Between May and September a continuous decrease of ozone concentration s is observed in the lower stratosphere at high latitudes in the north ern hemisphere. Low local ozone concentrations are correlated with hig h concentrations of NO and NO2, and HCl. A detailed photochemical box model and a two-dimensional chemical model initialized by the Halogen Occultation Experiment (HALOE) data are used to calculate ozone destru ction rates between 20 and 31 km altitude for different situations dur ing the observational periods in mid and late summer. The largest ozon e destruction rates are computed for ozone-rich midlatitude air masses that are transported to high latitudes reaching low Sun, but 24 hours per day sunlight conditions. It is shown that the observed summertime low ozone concentrations and much of the seasonal course of ozone is due to catalytic ozone destruction by NO and NO2, which become the mai n odd nitrogen compounds under these conditions.